For safety purposes, an increasing number of motor vehicles have monitoring systems that comprise sensors that are mounted on each of the wheels of the vehicle, dedicated to the measurement of parameters, such as pressure and/or temperature of the tires that equip these wheels, and intended to inform the driver of any abnormal variation of the measured parameter.
These monitoring systems are conventionally endowed with a sensor that is mounted on each of the wheels of the vehicle and that comprises a microprocessor and a radiofrequency emitter (or RF emitter), and a central unit for receiving signals that are emitted by the emitters (whereby this central unit is placed in the vehicle), comprising a computer that integrates a radiofrequency receiver (or RF receiver) that is connected to an antenna and that has an RSSI-type output (“Receiver Signal Strength Indicator” or literally an indicator of the power of the signal of the receiver) that can allow the analysis of the field level received by said receiver.
The major problem that such monitoring systems are required to resolve resides in the obligation of having to combine with each signal received by the receiver information that provides the position, in particular the longitudinal position, with regard to the front wheel assembly or the rear wheel assembly, of the sensor at the origin of this signal, whereby this obligation will last throughout the service life of the vehicle, i.e., having to be respected even after wheels are changed or more simply the positions of these wheels are reversed.
At present, this longitudinal positioning of the front wheel assembly/rear wheel assembly is obtained by comparing the field levels received from each emitter by applying the known power attenuation principle of electromagnetic waves that obey a law of reduction that is proportional to the square of the distance between emitter and receiver.
According to this principle, it was noted that a reliable differentiation between front and rear wheel assemblies requires that a margin be set between the field levels received for the purpose of taking into account:                on the one hand, the power tolerance of the sensors that are used for the purpose of preventing possible power variations during a wheel change,        on the other hand, the level difference required for a detectable differentiation between front wheel assembly and rear wheel assembly from a monitoring system standpoint.        
In practice, and as shown in FIG. 2 (in which the antenna appears with reference 9), the required necessary margin proves to be on the order of 10 decibels: 4.5 dB for taking into account the tolerance of the sensors, and 5.5 dB for the front wheel assembly (AV)/rear wheel assembly (AR) differentiation.
Actually, such a margin proves necessary for eliminating, for example, during a wheel change, a reduction in the level difference between front wheel assembly and rear wheel assembly, resulting from the mounting on the new wheel of an end sensor from the power standpoint (i.e., a sensor whose power is close to one of the values, minimum or maximum, of its tolerance margin). By way of example, FIG. 3 illustrates the result that is obtained during the replacement of a left front wheel by a left front wheel combined with a sensor with a high power limit, which justifies the value of margin required.
In practice, however, ensuring such a margin turns out to constitute a limiting factor on the possibilities of use of the corresponding tire monitoring process, because this high margin is very difficult to ensure on all vehicle platforms and leads to significant constraints on the architecture of these vehicles.